Degassing means the removal of gases, especially (i) gases from evaporated liquids like water or (ii) vapours that result from sublimating materials adhering to surfaces or (iii), in vacuum technology, substances that are outgassing from (bulk) material as soon as the surrounding pressure falls below its vapour pressure. In certain vacuum treatment processes, especially vacuum sputter coating processes degassing is an important process step, since residual gases may result in deteriorated adhesion of deposited layers or unwanted by-products in the deposits.
One differentiates between atmospheric and sub-atmospheric degassing. As the term suggests, sub atmospheric degassing takes place in an environment where the surrounding pressure can be lowered below atmospheric pressure.
It is known that degassing can be accelerated by heating the substrates thus enhancing the outgassing rate. This method may however have its limits for certain types of materials (e.g. plastics) or where the result of previous process steps could be (negatively) affected, such as melting solder bumps, warping of substrates or increased unwanted diffusion processes. Pump capacities may be improved to more quickly remove unwanted vapours and gases.
However the physics of the outgassing process itself remains the main limiting factor. In order to avoid that in an inline processing system with a sequence of defined process steps the degassing of a single substrate becomes the determining factor for the throughput, degassing is sometimes organized in batches. In other words, a plurality of substrates is being exposed jointly to an environment that assists the degassing. Such a batch degasser thus also acts as an intermediate storage for substrates
Consequently, there is a need for an apparatus for degassing substrates, shortly “degasser”, for (highly) outgassing substrates in a batch to enable longer outgassing times without the need to sacrifice throughput in subsequent processes.